An electrical load may appear to a power supply as a resistive impedance, an inductive impedance, a capacitive impedance, or a combination thereof. Ideally, when the current passing to the load is in phase with the voltage applied to or crossing the load, the power factor approaches one. Due to the nature of alternating current, the power factor of AC power can easily be less than one in certain situations (e.g., when the voltage is close to zero). In such situations, transmitted power/energy can be wasted (due to phase mismatch between current and voltage) and/or noise may be introduced into the power line. To reduce the noise to the power line caused by electrical loads and to improve the efficiency of power transmission, power supplies generally have power factor correction (PFC) circuitry to shape the input current waveform to follow the input voltage waveform. The closer the phase of the input current waveform follows the phase of the input voltage waveform, the more efficient the power conversion and the less noise is returned to the AC power line. The power factor, or PF, is a measure of this power conversion efficiency, and ideally, the PF for a given power converter should approach 1 under all conditions. When the PF does not approach 1, even under limited conditions, some portion of the transmitted energy is wasted, and current that should be passed onto a load may be returned, thereby introducing noise onto the power line.
FIG. 1 is a diagram of a conventional boost converter 10, in which an alternating current power supply AC is received at four-way rectifier 15. Input current Iin passes through inductor 20, and under certain operational conditions, a part of input current Iin passes through diode 50 (having a capacitor/filter 60 at its output) before being applied to load 70. Power factor controller 30 effectively controls the current flowing through inductor 20 by turning switch 40 on and off in response to an AC voltage-sensing input 12, a DC output voltage 72, a sensed power conversion current from a current detection inductor 25, and a feedback current 34. When switch 40 is on, a current 22 generally flows through inductor 20 (thereby storing some energy in inductor 20), then through switch 40 to ground. When switch 40 is off, a current 52 may flow through diode 50 and some charge may collect at capacitor 60, but generally, current flow 22 through inductor 20 is significantly reduced or even prevented.
FIG. 2 is a graph showing an AC voltage V into AC-DC converter 10. Input voltage V is the rectified half-sine wave of the AC waveform input. However, due to the on/off cycles of switch 40 (controlled by controller 30; see FIG. 1), the current waveform I in FIG. 2 has a sawtooth pattern. After passing such a sawtooth waveform I through a low-pass filter (e.g., high frequency bypass capacitor 17 in FIG. 1), the input current waveform resembles the input voltage AC at the input of rectifier 15, and the PF for the conversion approaches 1 under most conditions, particularly those conditions where the loading power is sufficiently high to allow an appreciable average input current to continuously pass through inductor 20. This is, in fact, known as the “average current mode” or “continuous mode” of operation for boost power converter 10.
The PFC for a given boost converter generally has two parameters defined by a specification: (1) PF, and (2) total harmonic distortion (or THD). THD refers to distortion caused generally by higher order harmonics (e.g., for a 60 Hz AC signal, distortion in the converted power signal caused by AC signals having a frequency of 120 Hz, 180 Hz, or other n*60 Hz value, where n is an integer of 2 or more). Generally, the higher the THD, the lower the efficiency. Such harmonics can saturate the transformer coils in boost converter 10 (e.g., in inductor 20). Moreover, if the THD is sufficiently high, noise can be fed back onto the AC power lines 12-14, a highly undesirable result from the perspective of a systems designer (e.g., of a power line network).
FIG. 3A shows a low-power and/or low-voltage portion 120 of the voltage and current waveforms of FIG. 2. The voltage waveform V is the voltage at the output of rectifier 15 (see FIG. 1), and the current waveform I is the input current Iin passing through inductor 20. When switch 40 in FIG. 1 is turned on at time t0, current I increases in a substantially linear manner, as shown by slope 122. Switch 40 is on for a period of time determined by controller 30, and at the end of this time (point 124 on the current waveform I in FIG. 2), switch 40 turns off and current I decreases in a substantially linear manner. Switch 40 then is turned on again by controller 30 (see FIG. 1) after a period of time ts−t0, also determined by controller 30.
When current I=0 (i.e., I0, the current value during “zero current period” 126 in FIG. 3A), the average current or continuous mode of operation has a potential distortion issue. The THD, no matter how low, cannot be controlled during the zero current period 126 of waveform portion 120 because there is no current flowing through inductor 20 of FIG. 1. This lack of THD control can have a dramatic effect on the THD specification number. The discontinuous mode of operation of boost power converter 10 occurs during those periods of time where switch 40 is turned on and off for lengths of time sufficient for zero current periods to appear, and the critical mode of operation occurs when current waveform I (see FIG. 3A) is at or near zero (I0). Those skilled in the art generally wish to maximize the amount of time that the inductor current Iin is above zero (see FIG. 1) and minimize the zero current periods (e.g., zero current period 126 of FIG. 3A).
As a result, the need in the art to turn switch 40 on as soon as possible when current I=0 has been long felt. Referring now to FIG. 3B, ideally, ts would be at the point in time when current I crosses I0 (the “I=0” axis), zero current period 126 would have a length as close to 0 units of time as possible, switch 40 (see FIG. 1) would be turned on essentially immediately by controller 30 (see FIG. 1) after current waveform portion 134 intersects I0 (see FIG. 3B), thereby causing current waveform portion 136 to increase essentially immediately after current waveform portion 134 intersects I0 and enabling current to flow through inductor 20 (see FIG. 1) substantially continuously. One generally avoids turning switch 40 on too soon (i.e., before current waveform portion 134 in FIG. 3B intersects I0), in order to avoid causing the average input current from increasing at too high a rate, which could cause the input current waveform phase to move out of alignment with the input voltage waveform phase.
There have been several approaches attempting to achieve results as close as possible to the ideal results shown in FIG. 3B. One such approach involves trying to detect directly the input current Iin flowing through inductor 20 (see FIG. 1). One widely used technique employs a second inductor coil 25 to sense the current Iin flowing through inductor 20 in a manner similar to the function of a transformer. However, this approach suffers from the inevitable latency that all transformer coils experience when sensing a current in another coil, necessarily introducing some positive length of time in the zero current period 126 (see FIG. 3A) and introducing some noise back into the AC power line 12-14. Also, the second inductor coil 25 adds some expense to manufacturing controller 30 and necessitates at least one dedicated differential pin on controller 30 to receive information from second inductor coil 25.
Alternatively, one could try to sense the current at node 34 in FIG. 1. However, the current and voltage values at node 34 are relatively low in the critical mode of operation, thereby increasing the relative error in current determinations at node 34 to a level where such determinations may not be sufficiently accurate for commercially successful applications. Also, determining the current at node 34 would require controller 30 to have a relatively high sampling rate (i.e., >>1 sample taken every 1/[ts−t0] seconds) in the critical mode, and the sampling resolution should be relatively high to avoid turning switch 40 on too fast or too slow.